In September 2009, the President announced a revised approach to missile defense in Europe called EPAA, which consists of phases of increasing capability to be deployed in the 2011, 2015, and 2018 time frames. EPAA serves as the U.S. contribution to the North Atlantic Treaty Organization's (NATO) mission to protect alliance populations, territory, and forces against ballistic missile threats. As originally announced, EPAA included a fourth phase that was expected to add U.S. homeland defense and expanded regional defense in the 2020 time frame. In March 2013, the Secretary of Defense canceled Phase Four, due, in part, to development delays with a key element of this phase.